The field of the invention relates to temperature control of emission control devices coupled to direct injection spark ignition engines.
In direct injection spark ignition engines, the engine operates at or near wide open throttle during stratified air/fuel operation in which the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to conventional three way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three way catalytic converter to further reduce NOx.
The inventors herein have recognized that lean air/fuel operation may cause the catalytic converter and NOx trap to operate inefficiently. The inventors herein have also recognized numerous disadvantages in prior approached to heating catalytic converters. For example, retarding ignition timing is inefficient because engine surfaces such as cylinder walls will be heated in addition to heating exhaust gases. Further, the amount of heat which may be generated by retarding ignition timing is limited. And, retarding ignition timing is not feasible during stratified mode operation. The inventors also recognize that use of a variable length exhaust pipes to control NOx trap temperature is limited i8n the range of temperature change and not desirable because of cost and packaging.